Seyfert Spectra

OSSE has shown conclusively, for the first time, that the high-energy emission
from the Seyfert class of Active Galactic Nuclei exhibits a high-energy cutoff
above a few hundred keV. This is seen in NGC 4151, the brightest radio-quiet
AGN in the OSSE energy band, as well as the average of all Seyfert AGNs detected
by OSSE. Above 50 keV an exponentially-cutoff power-law spectrum, N(E)
µ EG exp(E/Ec), is an adequate description of the observed
spectrum. For the average of all NGC 4151 observations, we find a power-law
index, G = 1.83 ±
0.03, and cutoff energy, Ec = 96 ±
43 keV; for the average Seyfert we find G
= 1.5 ± 0.3 and Ec = 120
± 37 keV. Combining the OSSE data with lower
energy observations provides the best insight to the nature of these sources.
These analyses of OSSE, Ginga and EXOSAT data suggest that the best physical
model for radio-quiet Seyferts is mildly relativistic thermal Comptonization
(including a Compton-reflection component, required by the X-ray data). The
best fit plasma parameters are kT ~ 100 keV and optical depth,
t ~ 1.

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